1. Field of the Invention
This invention relates to fluid driven downhole motors used in the oil and gas industry and, more particularly, to an apparatus for preventing stalling of such motors caused by excessive resistance during drilling or milling.
2. Description of the Related Art
A downhole motor may be utilized at some point during the drilling or producing life of an oil well to aid in the removal of solid materials from the well. The motor can be incorporated into a drill string to provide rotational torque to a drill bit or other similar downhole drilling or milling device without the need to rotate the entire drill string. The motor is attached near the bottom end of the drill string. The drilling device is attached at or near the bottom of the motor. The motor and drilling device are deployed into the well, and fluid is pumped through the drill string and into the body of the motor, driving the lower section of the motor and causing it to rotate. The drilling device rotates along with the lower section of the motor to engage and loosen materials within the well bore.
During the course of the drilling operation, the operator must maintain an optimum weight on the drilling device to facilitate drill penetration yet prevent motor stall. Motor stall can result from shifting and accumulation of debris near the drilling device, which can cause resistance to motor rotation and back torque by the motor. Motor stalling can cause increased pressure drop across the motor and diminish the life of the motor. A weight indicator reading at the surface can be used to monitor weight on the drilling device. Motor stall can be monitored by a pump pressure gauge. When stalled, a pump pressure reading in excess of the maximum recommended pump pressure at a given rate will be observed. Once a motor becomes stalled, the operator must hoist the work string to decrease weight on the drilling device until motor rotation resumes, then lower the work string to optimize weight on the bit. This process is time consuming and lengthens the drill phase.
Due to the limited torque output of downhole motors, stalling can be frequent. Also, maintaining optimal weight on the bit is difficult, as the operator must attempt to lower the work string at a rate concurrent with the removal of drilled off material. In addition, cuttings from the drilled material can shift and gather under the drill bit, increasing back torque or altering the weight on the bit and causing a stall. The objective in a drilling operation with a downhole motor is to penetrate as quickly as possible without stalling the motor.